1. Field of the Invention
This invention relates to water retention aids useful in paper coating compositions.
2. Description of the Prior Art
Paper coating compositions, also known as coating colors, are applied to paper to improve certain of its properties such as brightness, smoothness, gloss, ink receptivity and the like. They are applied to the paper as water suspensions. Consequently, one of the concerns regarding use of the coating compositions is water retention, i.e., the ability to retain the water in the coating after application to the paper thereby reducing its migration to the paper. Subsequently, the water is removed from the coated paper by drying. In order to control release of the water, various water soluble polymers are added to the coating composition. They are known as water retention aids. See, for example, U.S. Pat. No. 4,425,452 --Nakata et al, Jan. 10, 1984, which points out that coating colors may contain auxiliary components such as water retention aids.
Examples of water retention aids are carboxy methyl cellulose, sodium alginate and synthetic polymers such as alkali swellable acrylate latex and polyacrylic acid. The carboxy methyl cellulose and sodium alginate are powders requiring dissolution in water before use. Aside from this additional preparation step, risk is run that undissolved particles remain in the paper coating composition which will mar the coating finish. In the case of alkali swellable acrylate latex, it is necessary to introduce ammonia or sodium hydroxide to put it in solution. Aside from this added step, handling ammonia and sodium hydroxide is disadvantageous. Regarding polyacrylic acid and alkali swellable acrylate latex, they have been found to increase the viscosity of the paper coating excessively.